Carboxylic acids, such as propionic acid, are important intermediates for the synthesis of a number of oxygenates that find applications in herbicides, food preservatives, plastics, plasticizers, and cosmetics.
Various methods are known for the production of carboxylic acids. Taking as an example propionic acid, one commercial method relies on the liquid phase hydrocarboxylation of ethylene. Although there are a number of companies that have claimed such a process, including Shell (EP 0 495 547) and Monsanto (U.S. Pat. No. 3,816,490), BASF (U.S. Pat. No. 5,866,716) is the sole manufacturer that exploits this direct liquid phase route. In this process ethylene, CO and water are converted directly into propionic acid in the presence of a highly toxic Ni(CO)4 catalyst at harsh reaction conditions (250-320° C., 100-300 bar).
A second liquid phase method to produce carboxylic acids uses olefin hydroformylation, followed by oxidation of the aldehyde to produce the carboxylic acid. In this commercially practiced, two reaction step, process to produce propionic acid, propanal is produced in the first step via the hydroformylation of ethylene, and in a second step propanal is oxidized to propionic acid (“Ullmann's Encyclopedia of Industrial Chemistry” Vol. 30, pp. 295-311 (2012)).
Another route to produce carboxylic acids is the direct oxidation of hydrocarbons (“Ullmann's Encyclopedia of Industrial Chemistry” Vol. 30, pp. 295-311 (2012)). Direct oxidation of hydrocarbons can also be used to produce propionic acid as a by-product during acetic acid synthesis from naphtha (“Ullmann's Encyclopedia of Industrial Chemistry” Vol. 30, pp. 295-311 (2012)).
The liquid phase single-step hydrocarboxylation of ethylene has an advantage in ethylene yield compared to the two-step hydroformylation/oxidation route; however, it has found limited industrial use because of the cost and risk associated with operating a high pressure reactor that uses a corrosive and toxic nickel carbonyl catalyst.
The processes listed above refer to reactions in the liquid phase. The open literature on gas phase hydrocarboxylation is limited. Early work described the formation of a carboxylic acid via mixing steam with CO and an olefin. Examples of catalysts are charcoal (U.S. Pat. No. 2,089,903), ZnCl (U.S. Pat. No. 1,924,767) and W oxides (U.S. Pat. No. 2,008,348), and in all cases the catalysts were used in combination with metal halides. Although these works indicate a pressure range between 25 and 900 atm, the examples are performed at 600-700 atm.
U.S. Pat. No. 3,501,518 discloses that the carbonylation reaction can by activated by Pd sulfide. The reaction is performed in the liquid phase at a temperature range of 30-180° C. at a pressure of 5-100 MPa (49-987 atm) and requires the addition of halides or co-catalysts such as acids and an organic phosphine or nitrile.
In view of the deficiencies of the prior art methods, it would be desirable to have an alternative catalyst system and process for the gas phase, single step hydrocarboxylation of olefins to carboxylic acids.